Because everything made by adventurers (Quests, Dungeons, Raids, etc.) in WoW, sadly becomes lore and in the Burning Crusade-Expansion Illidan became a Raid-boss that should be killed in a quest, though I haven't been in the Black Temple yet... He has been killed, but he's still alive anyway, so he can soon be killed again by other adventurers, like mine... (LOL!)